1. Field of the Invention
The present invention generally relates to an apparatus and process for mixing a gas and a liquid and, more particularly, relates to the field of aeration associated with waste-water treatment and the control of water-pollution wherein waste-water is aerated or mixed with other gases.
2. Description of the Prior Art
Pollution and the control of pollution have become quite important in recent years. In the past, pollution control was concerned primarily with problems caused by domestic and the simpler waste of industry. In this respect, control was aimed principally toward protecting downstream public water supplies and stopping or preventing nuisance conditions.
It is no longer true that pollution problems are either local in extent and control or a matter solely of local consideration. Thus, national growth and change have altered this picture considerably and the stopping of pollution is an ever-increasing national problem.
The growth of the population has far outdistanced progress in stopping pollution and the increased production of goods has greatly increased the amounts of common industrial wastes. Conventional biological waste treatment processes are hard-pressed to hold the pollution line, and for a growing number of larger cities, conventional processes are no longer adequate. Accordingly, a great deal of study and research has been conducted to develop more efficient and better processes for treatment of waste-waters.
Municipal wastes usually contain oxygen-consuming pollutants; synthetic organic chemicals, such as detergents; sediments; and other types of pollutants. Industrial wastes could contain a wide diversity of constitutents and, in addition, substantial amounts of heat from cooling processes. Also, water that drains off the land usually contains quantities of organic matter in addition to sediments and may also contain radioactive substances and other pollutants washed from the atmosphere from vegetation, etc. Accordingly, efficient and effective waste-water treatment processes are needed which can remove and eliminate each of these various types of pollutants.
At this time, wastes are generally treated by two basic methods, called primary and secondary methods or stages, with, optionally, a tertiary stage in the treatments. The primary method or stage is one in which the raw waste material is treated to remove the settleable solids, i.e., clay, bits of organic wastes and oil droplets, etc. This state is one which is mostly mechanical. For example, as the sewage enters a plant for primary treatment, it flows through a screen which removes large floating objects, such as rags and sticks, that may clog pumps and small pipes. After the sewage has been screened, it passes through a grit chamber where sand, grit, cinders and small stones are allowed to settle to the bottom. Suspended solids are then removed in a settling tank. Other mechanical treatments which can be utilized in the primary method or stage of waste treatment include, for example, flotation methods, coagulation and flocculation. All of these methods are mechanical and comprise a method for removing the suspended particles in the raw sewage. The secondary method or stage in waste-water treatment is basically biological in nature and can remove up to 95% and sometimes more of the organic matter in the sewage by making use of bacteria. Such secondary treatment is designed to remove dissolve materials such as sugars, starches and phenols, which are susceptible to biological treatment. The two principal types of secondary treatment are the trickling filter and the activated sludge process. In addition, use has been made of stabilization ponds and aeration lagoons, all these methods in the secondary stage of treatment working on the same basic principle, i.e., the enchanced biological degradation or consumption of the organic compounds.
Aeration is the mass transfer process by which a gas is transferred into a subsaturated liquid. The purpose of aeration in the activated sludge system of waste-water treatment is to efficiently dissolve oxygen into mixed liquor (mixed sewage or other waste-water influent and recycled activated sludge) as well as to keep the mixed-liquor suspended solids (MLSS) in suspension. In the present state of the art, most of the aeration of waste-water is performed by mechanical surface aerators, air diffusers or sparge-turbines.
The mechanical surface aerators entrain the air in the water on, over or close to the surface of water. There is a water circulation pattern set-up which is from the lower region of the vessel through a central draft tube located below the turbine, into the turbine or aeration unit.
Sparging air into the water in most cases is accomplished by forcing air to flow through a conduit which has small holes through its wall. The air passes through the holes in the conduit wall into the waste-water. Another method of sparging is to use a porous ceramic shape and force the air through the porous ceramic into the water.
In the activated sludge method, after the sewage leaves the settling tanks of the primary treatment facilities, it is pumped to an aeration tank in which it is mixed with previously activated sludge and subjected to aeration for up to several hours. During the time in which the sewage, bacteria and air are in contact, the bacteria break down the organic matter in the sewage. The sewage leaving the aeration tank in the form of a mixed liquor thereafter flows to a final sedimentation tank to remove the solids produced in the aeration tank. The basic secondary treatment of the sewage can then be followed by disinfection of the effluent.
The sludge which has been previously activated is returned to the aeration tank for mixing with new sewage and additional amounts of water. This allows the activated sludge process of the secondary treatment stage to be conducted continuously with recycling of the activating sludge.
Practically all sewage treatment plants in operation or being built today make use of some form of activated sludge treatment process. The process is similar to nature's way of recycling materials for reuse but with an effort made to greatly accelerate the treatment. The process is basically bio-conversion and, with proper clarification, the treated water is released from the plant as the effluent to rejoin the environment via rivers and the sea. Excess or wasted sludge not used or needed in the ongoing oxidation process must be further digested and eventually disposed of as inert matter in land fills, sea dumps, farming, etc.
While the activated sludge process has many advantages when compared with other conventional techniques, the activated sludge process has certain disadvantages also. For example, the activated sludge process is more costly to operate than, for example, the trickling filter method or the reactor process. In addition, conventional activated sludge processes sometimes lose their effectiveness when faced with difficult industrial wastes. From a practical viewpoint, large, slow-speed turbines throw blades causing imbalance. Rags caught in the turbine are more often than not the cause of this problem. Pump-type surface aerators have a tendency to freeze up on cold climates. Plugging of diffusers constitutes a continuous maintenance problem for the operating staff. Accordingly, while the activated sludge process is the best available secondary treatment process, it is not wholly satisfactory.
The conventionally used processes for contacting the sewage with air have shown to be quite inefficient. For example, it has been discovered that the foregoing mechanical and forced air methods of waste-water treatment have an oxygen transfer efficiency of less than about 10%, i.e., less than 10% of the oxygen in the air is transferred to the sewage. In addition, it has been discovered that, in terms of pounds of oxygen transferred to the sewage or absorbed by the sewage per horsepower hour, the mechanical method and forced air method allow an absorption of less than about 3 pounds of oxygen per horsepower hour. Accordingly, it can be seen that, if the transfer efficiency of the oxygen transferred or absorbed by the sewage can be increased, the activated sludge process can be substantially improved. The present invention was developed as the result of efforts to eliminate the foregoing problems and disadvantages of conventional processes.
After the secondary treatment stage of the method, the effluent is subjected to optional tertiary treatment operations designed to eliminate residual dissolved organic and inorganic compounds. These materials can be removed by electrodialysis or ion exchange and such operations as distillation, freezing or other desalinization-type techniques. In addition, activated carbon has been used. Also, municipal waste-waters generally contain many organic materials which are only partially removed by the foregoing conventional treatment methods. A good example of these types of organic materials are detergents. Oxidants such as ozone and chlorine have been used to improve the taste and odor qualities or to disinfect municipal drinking water. These materials improve the quality of the water by destroying or altering the structure of the chemicals in the water and provide disinfection by destroying bacteria.
The use of such disinfectants and oxidants in the treatment of waste has not been as effective as possible due to inadequate mixing. However, a more efficient method of contacting the ozone or chlorine with the waste could provide an effective means for disinfecting sewage. Accordingly, the present invention has been developed as the result of research in connection with such a problem.
With this understanding of sewage treatment, it quickly becomes apparent that the basic question as to what bio-conversion system to use centers on the degree to which the system can remove or oxidize the biodegradable materials present while minimizing the wasted sludge produced and do this for the least capital expenditure and the minimum operating cost. The key to securing this optimum system lies in effectively and efficiently conducting the bio-conversion process to consume the "food". Thorough and complete mixing of the food (from the sewage or sludge) and oxygen (from the air) with the bacteria in the water is a necessity in such a process. It, therefore, becomes imperative to use a most effective and energy-efficient mixing process.
The complete mixing of activated sludge is a basic process in which the incoming raw waste is completely mixed with the entire contents of the aeration tank so that the degree of degradation of the organic material does not vary from one portion of the tank to the other. Accordingly, it is quite advantageous if the mixing of the air with the sewage and absorption of the same can be effective so as to be uniform over and within the entire tank. This has been achieved with efficiency in accordance with the process of the present invention.
In the activated sludge treatment, the stabilization or degradation of the organic matter is brought about by the biochemical activities of the microorganisms in the system. The bacteria are primarily responsible for the stabilization of the organic matter in activated sludge systems, the bacteria removing organic matter from wastes in order that they might obtain sufficient energy to remain alive, to produce new cells and to obtain the chemical components for the new cells. The bacteria present in the sewage must have oxygen to do their part in breaking down the sewage; accordingly, oxygen must be introduced into the activated sludge process in order to bring about the necessary stabilization of the organic matter. It is the ineffective transfer of oxygen to the sewage that has brought about the previously described defects and disadvantages of previous proposals.
In its broadest aspects, the principles of the aeration mixing process and various features of the aeration mixing apparatus of the present invention are generally described in U.S. Pat. Nos. 3,334,868 and 3,661,364, to James R. Lage. However, in the environment of the present invention, particularly as it relates to waste-water treatment, especially aeration, it was necessary to make many modifications and adjustments of the basic process and apparatus in order to realize the potentials and benefits possible in such gas-liquid mixing system.
The gas-liquid mixing process of this invention can be utilized with any substances capable of flowing but, in biological waste-water treatment, it is used principally to mix liquid and gas. More specifically, the process may be used in the high rate aeration, bio-oxidation, digestion and disinfection stages of sewage or other waste-water treatment. The gas involved in all of these mixing stages is air or other oxygen-containing gas, including pure oxygen, oxygen enriched air, oxygen-nitrogen mixtures, ets. except for the disinfection state, which employs chlorine or ozone.